1. Technical Field
This patent application relates, in general, to suppressing electromagnetic radiation in and around data processing systems.
2. Description of the Related Art
Data processing system components (e.g., personal computer system components such as integrated circuits) are typically mounted on or integrated within printed circuit boards. During operation of a data processing system having one or more printed circuit boards, the data processing system components cause electric currents to be generated on or in the printed circuit boards and on or in the associated data processing system components. Such electric currents often result in electromagnetic energy being radiated.
Electromagnetic radiation can interfere with data processing system operation (in which case the electromagnetic radiation is referred to as electromagnetic interference (EMI)). Accordingly, efforts are made within the art to shield printed circuit boards and/or their associated data processing system components from electromagnetic radiation. Conventionally, such shielding is accomplished via sheet metal structures used to enclose all or part of the printed circuit board and data processing system components utilized in data processing systems. These sheet metal shielding structures block electromagnetic energy and thereby serve two functions: (1) they shield printed circuit boards and/or data processing system components external to the shielding structures from electromagnetic radiation emanating from printed circuit boards and/or data processing system components internal to the sheet metal shielding structures, and (2) they shield printed circuit boards and/or data processing system components internal to the sheet metal shielding structures from electromagnetic radiation emanating from sources external to the sheet metal shielding structures.
The ideal situation is to have completely sealed sheet shielding structures, which provide virtually total electromagnetic radiation shielding. Unfortunately, this is generally not practicable in current data processing system environments, because most modern data processing system components require active air, water, or other cooling. For example, modern microprocessors generate great amounts of heat energy, and are only rated to function accurately up to a specified case (a plastic or ceramic shell enclosing the microprocessor) temperature, and without active cooling, it is likely that the microprocessors will function incorrectly or fail. Accordingly, it is generally not practicable to enclose a microprocessor, or any other data processing system component, in a completely sealed sheet metal shielding structure since such a sealed sheet metal shielding structure traps heat.
As a compromise, current practice is to use a perforated structure (e.g., a sheet metal structure with holes) which allows some air flow through the structure (via the perforations, or holes) and which also provides some electromagnetic shielding (via the presence of the sheet metal conductor). Historically, designs of such perforated structures have typically been derived by a trial and error process involving (1) formation of a perforated sheet metal structure, (2) employment of the perforated sheet metal structure within a data processing system, (3) measurement of heat dissipation and emitted radiation characteristics of the so-employed structure, and (4) successive modification of the structure in response to such measurements until a structure having acceptable characteristics was obtained.
More recent designs of sheet metal structures have utilized less of a trial and error approach. For example, at least one recent design has advocated using two layers of slotted metallic conductive material, with the slots of the first layer oriented transverse to the slots of the second layer material. The teaching is that the material, so arranged, will block both vertically and horizontally polarized electromagnetic waveforms while allowing ventilation. While this design works well in theory, in practice it has proved sub-optimum. As another example, at least one other recent shielding design has advocated utilizing sheet metal structures in which one or more waveguide below cutoff structures are resident. A waveguide is a hollow structure, formed from a low resistivity conductive material (typically metal), which only allows electromagnetic energy waveforms above a certain frequency (known in the art as the "cutoff frequency") to propagate through the hollow portion of the waveguide. Because the waveguide is a hollow structure, air can transit the structure, thereby providing cooling in a fashion analogous to the perforated shielding discussed above. In the waveguide below cutoff scheme, the waveguide is fabricated such that it functions efficiently as a waveguide only above a cutoff frequency, where the cutoff frequency is sufficiently above the frequency of the electromagnetic energy waveforms likely to be generated by printed circuit boards or other data processing system components.